Well Shark Robot is a giant store with lots of different artists selling their merchandise on there. Shark Robot makes a tiny bit off of each purchase. That's what's going to Endling this Month. And the commissions the artists normally make off each purchase still goes to that artist. It's not like a Snafu donation can make VG Cat's/Yugioh Abridged/Staccatto/Wapsi/etc donate all their money to Endling for instance.
An Endling item he get's his normal commission AND he get's the money Shark Robot is donating to him which would normally be Shark Robot's share. So it's like way better to buy something that Endling did but it ALL helps.
And as a heads up anything from Snafu goes more to Endling than the other store items would. And the Yugioh Abridged is donating an extra dollar from everything they sell to Endling so that's the next best stuff to get if you want to help out. I'm not sure if the others will want to or not totally up to them. Honestly I haven't asked.
So as of now best way to get money to Endling
Endling Item>Snafu Item>Yugioh Item>All other Shark Robot items.
Honestly though it all helps plenty. Hopefully that explains the difference